Process of making zinc borate and fire-retarding compositions thereof

ABSTRACT

Zinc borate compositions having a ZnO:B 2  O 3  ratio of 4:1. The hydrated zinc borate 4ZnO.B 2  O 3 .H 2  O and anhydrous zinc borate 4ZnO.B 2  O 3  are especially useful as fire-retardants and smoke-suppressant additives for polymeric compositions.

This invention relates to improved zinc borate compositions, and moreparticularly, this invention provides a new hydrated zinc borate havinga high dehydration temperature which offers significant advantages forcompounding with plastics and rubbers at elevated temperatures. Theanhydrous form of the zinc borate is also provided, offering advantagesfor compounding at even higher temperatures.

BACKGROUND OF THE INVENTION

Many different hydrated zinc borates are known and several findcommercial application as fire retardants and smoke suppressants forvarious polymers. Hydrated zinc borates are also used as anti-corrosivepigments for coatings and have demonstrated fungistatic andbacteriostatic properties which find many applications.

The known hydrated zinc borates include ZnO.B₂ O₃.H₂ O, 2ZnO.3B₂O₃.3.5H₂ O, 2ZnO.3B₂ O₃.7H₂ O, 3ZnO.5B₂ O₃.14H₂ O (sometimes designated2ZnO.3B₂ O₃.9H₂ O), ZnO.B₂ O₃.2H₂ O, ZnO.5B₂ O₃.4.5H₂ O, 2ZnO.3B₂ O₃.3H₂O, and 6ZnO.5B₂ O₃.3H₂ O. See Supplement to Mellor's ComprehensiveTreatise on Inorganic and Theoretical Chemistry, Vol. V, Part A, Pages577-578, Longman Group Ltd (1980). See also pages 572-6 for a review ofanhydrous zinc borates. Several of these zinc borate hydrates havecommercial importance, especially as fire retardants and smokesuppressants for polymers, rubbers and coatings. The relatively highdehydration temperature (about 290° C.) of 2ZnO.3B₂ O₃.3.5H₂ O offers asignificant advantage over other commercially available hydrated zincborates since elevated temperatures are required for compounding manyplastics and rubbers. However, some of the recently developedengineering plastics require processing at even higher temperatures inthe range of about 300°-400° C., and it is desirable to have a zincborate which has an even higher dehydration temperature.

SUMMARY OF THE INVENTION

The present invention provides a new crystalline, hydrated zinc boratehaving a relatively high dehydration temperature which makes itespecially useful for use in polymers requiring processing at hightemperatures. The anhydrous form of the zinc borate is also provided.

DESCRIPTION OF THE INVENTION

The hydrated zinc borate of this invention has the formula 4ZnO.B₂ O₃.H₂O. It is a crystalline solid having very slight water solubility andhaving a dehydration temperature which begins at about 415° C. with arapid loss occurring above 425° C. Such a high dehydration temperaturemakes this composition especially useful as an additive for polymersrequiring high processing temperatures such as the polysulfones,polyamide-imides, polyketones, polyetherketones and polyarylates. Thewater solubility of the compound is considerably lower than that ofother known hydrated zinc borates.

The zinc borate hydrate of this invention can be readily prepared by avariety of methods. The presently preferred method of productioncomprises the reaction of zinc oxide with a near stoichiometric amountof boric acid (2:1 mole ratio) in water at an elevated temperature,according to the equation

    4ZnO+2B(OH).sub.3 →4ZnO.B.sub.2 O.sub.3. H.sub.2 O+2H.sub.2 O.

The reaction preferably takes place near the boiling point of themixture and is promoted by the presence of previously prepared productseed. The concentration of the initial reaction mixture should begreater than about 5% by weight of starting reagents in order to providea reasonably rapid reaction rate. Preferably, reaction mixtures in the10-20% by weight range are employed, since they require only a few hoursof refluxing in water to go to completion. It is also preferred that aslight molar excess (approximately 5%) of boric acid is used in order toprovide complete consumption of zinc oxide in the reaction. The desiredhydrated zinc borate product is readily separated from the cooledreaction solution by filtration and dried to give the desiredcrystalline product. It has also been found that more consistent resultsare obtained when boric acid is added to the zinc oxide in boiling waterin at least two separate portions, thereby maintaining the pH of thereaction mixture above about 5.5, and the reaction mixture isefficiently stirred or mixed during the reaction period.

Other methods for preparing the hydrated zinc borate of this inventioninclude hydrolysis of the zinc borate 2ZnO.3B₂ O₃.3.5H₂ O in a refluxingaqueous slurry. It has been found that the concentration of the startingmaterial in the aqueous slurry must be less than about 5% in order toproduce a complete reaction. At least five days of continuous reflux atatmospheric pressure of a 5% slurry of starting material is required togive complete hydrolysis. Reflux of the reaction mixture containing freeboric acid for an excessive period of time (such as for one month) willconvert the zinc borate to the compound 6ZnO.5B₂ O₃.3H₂ O which waspreviously reported as a product of the hydrothermal reaction of zincoxide and boric acid at 165° C. See Lehmann, H.-A. et al, Zeitschriftfur Anorganische and Allgemeine Chemie, 1967, 354, Page 37.

The zinc borate of this invention may also be prepared by reacting2ZnO.3B₂ O₃.3.5H₂ O with a stoichiometric amount of zinc oxide inrefluxing water. This reaction is also facilitated by the presence ofseed crystals of previously prepared zinc borate 4ZnO.B₂ O₃.H₂ O. Insome cases, the reaction appears to be catalyzed by the presence of zincion such as supplied by a small amount of zinc chloride or zinc sulfate.

A fourth method for preparing the 4ZnO.B₂ O₃.H₂ O of this invention isby reaction of sodium tetraborate with a zinc salt such as zinc sulfateand zinc oxide in boiling water according to the equation

    Na.sub.2 B.sub.4 O.sub.7.5H.sub.2 O+ZnSO.sub.4.7H.sub.2 O+7H.sub.2 O+7ZnO→2(4ZnO.B.sub.2 O.sub.3.H.sub.2 O)+Na.sub.2 SO.sub.4 +10H.sub.2 O.

The presence of about 5% seed product facilitates the reaction whichgoes to completion within a few hours.

EXAMPLES

The following examples illustrate preparation and use of the zincborates of this invention.

EXAMPLE I

200 Grams of 2ZnO.3B₂ O₃.3.5H₂ O (0.460 mol) was added to 4.5 L ofdeionized (DI) water and boiled under reflux for six days. During thistime the pH of the reaction solution gradually decreased from 7.8 toabout 4.5. The reaction slurry was then filtered, washed with DI waterand air dried to give 92.2 g (97% yield) of product, 4ZnO.B₂ O₃.H₂ Ohaving the following analysis:

    ______________________________________                                                      Calc.                                                                              Found                                                      ______________________________________                                        ZnO             78.79  78.35                                                  B.sub.2 O.sub.3 16.84  17.04                                                  H.sub.2 O        4.36   4.88                                                  ______________________________________                                    

Since slight variations in analyses can be expected, a typicalcomposition of the hydrated zinc borate of this invention can be definedas 3.9-4.1(ZnO).0.9-1.1(B₂ O₃).0.8-1.2(H₂ O).

The XRD pattern for the product, set out below, bears no similarity toany known zinc borate compound. The following is the characteristicX-ray diffraction pattern for the hydrated zinc borate.

    ______________________________________                                        2-THETA/degrees D/degrees INTENSITY                                           ______________________________________                                        22.21           3.998     100                                                 18.78           4.721     94                                                  28.44           3.133     58                                                  36.31           2.472     55                                                  31.64           2.826     39                                                  21.91           4.053     35                                                  37.51           2.396     32                                                  33.82           2.648     31                                                  37.27           2.410     23                                                  32.67           2.739     20                                                  42.86           2.108     19                                                  40.65           2.218     19                                                  55.68           1.650     17                                                  48.97           1.858     16                                                  23.91           3.718     13                                                  ______________________________________                                    

A portion of the filtrate was evaporated to dryness to give acrystalline solid which was identified as boric acid by its XRD pattern.

EXAMPLE II

A 5-L flask was charged with 100 grams (0.23 mole) of the zinc borate2ZnO.3B₂ O₃.3.5H₂ O, 74.9 grams (0.92 mol) of ZnO, 7.2 g. (17 mmol) ofpreviously prepared seed product, and 2.0 L DI water. To this slurry wasadded 0.5 g of ZnCl₂ (3.7 mmol). The mixture was boiled under reflux forsix hours with mechanical stirring.

The reaction was then cooled, filtered, and the product air dried togive 146.4 g (98% yield) of 4ZnO.B₂ O₃.H₂ O, identified by its XRDpattern.

EXAMPLE III

A 5-L round bottom flask was charged with 488.4 g. of ZnO (6 mol) and3.5 L of DI water. This slurry was brought to a boil and 28.0 g. ofpreviously prepared seed (0.07 mol) and 97.4 g of boric acid (1.58 mol)was added. After boiling this mixture under reflux with stirring for 2.5hours, another portion of boric acid (97.4 g.; 1.58 mol) was added.After refluxing with stirring for another 2.5 hours, the reactionmixture was cooled and filtered. The solid product was washed with DIwater and air dried to give 629.2 g (97% yield) of 4ZnO.B₂ O₃.H₂ O.

EXAMPLE IV

Sodium tetraborate pentahydrate (45.9 g.; 0.158 mol) was dissolved in1.0 L of hot DI water in a 5-L flask. To this solution was added 43.1 gof ZnSO₄.H₂ O (0.15 mol) dissolved in 250 mL water. A white precipitateformed immediately. This mixture was brought to a boil and ZnO (85.5 g.;1.05 mol) and 6.2 g of previously prepared seed (15 mmol) were added.The reaction mixture was boiled under reflux for six hours. The reactionwas cooled, filtered, washed with water and air dried to give 132.7 g(97% yield) of 4ZnO.B₂ O₃.H₂ O containing some residual zinc oxide.

Zinc borate 4ZnO.B₂ O₃

Zinc borate 4ZnO.B₂ O₃ can be prepared by the dehydration of thehydrated zinc borate 4ZnO.B₂ O₃.H₂ O. The dehydration is readilyaccomplished by heating the hydrated borate at a temperature above 415°C. for a period of time sufficient to remove essentially all water.Generally heating at a temperature in the range of about 500°to 550° C.for about 3 to 5 hours will produce good yields of the desired 4ZnO.B₂O₃.

The zinc borate 4ZnO.B₂ O₃ is non-hygroscopic and is resistant torehydration even under high humidity conditions. This offers asignificant advantage over many other anhydrous metal borate compoundswhich are often appreciably hygroscopic.

EXAMPLE V

A 50.0 gram (0.12 mol) sample of the zinc borate hydrate of thisinvention was heated in a furnace at 500°-550° C. for about 4 hours.This resulted in a weight loss of 2.2 grams, corresponding to the lossof 0.12 mol of water.

To test for moisture reabsorption, a sample of the resulting anhydrouszinc borate was placed in an open container in a humidity chambermaintained at 90% relative humidity and 90° F. for one month. After thistime, a sample of this material was subjected to thermogravimetricanalysis (TGA). Less than 0.1% weight loss was detected upon slowlyheating the material over a period of about 2 hours from roomtemperature to 700° C., indicating that very little moisture wasabsorbed during prolonged storage under high humidity conditions.Furthermore, no significant weight loss was found when this material washeated continuously at 400° C.

The zinc borate compounds of this invention are useful as fire retardantand smoke suppressant additives for a wide range of organic polymercompositions. The polymer systems include the well-known polymers,rubbers and coatings compositions. Examples of such compositions arepolyvinyl chloride (flexible and rigid), nylons, polyolefins (e.g.polyethylene, polypropylene, and ethylene-propylene elastomers such asEPM and EPOM), chlorinated polyolefins, ethylene vinyl acetate,acrylates and acrylic copolymers, polyurethanes (flexible and rigid),polystyrenes, polybutadiene, polyesters, styrene-butadiene rubbers,acrylonitrilebutadiene-styrene (ABS), polysulfones, silicones, neoprene,fluoroelastomers (e.g. ETFE and FEP), cellulosics, polyphenylene oxide,polyethers, polyether imides, polyetherketones, polyethersulfones,epoxies, polycarbonates, phenolics, polyarylates, polyamides,melamine-formaldehyde and alloys and blends of the above.

The zinc borate is incorporated in the polymer systems at levels of fromabout 1 to 40 parts by weight of additive per 100 parts of resin (phr).Preferably, about 2 to 20 phr is added for best fire retardancy. Thepolymer systems may also contain other additives which areconventionally used in such compositions, including antimony oxide,alumnia trihydrate, stabilizers, plasticizers, halogenating agents,fillers such as calcium and magnesium carbonates, pigments, etc.

The following examples are illustrative of polymer compositionscontaining the zinc borates of this invention.

EXAMPLE VI

Flexible polyvinyl chloride compositions were prepared with variableamounts of the zinc borates 4ZnO.B₂ O₃.H₂ O and 4ZnO.B₂ O₃. Theformulations were as follows in which parts are by weight.

                  TABLE I                                                         ______________________________________                                                  Formulation No.                                                                 1      2      3    4    5    6    7                               ______________________________________                                        GEON 30 (PVC)                                                                             100    100    100  100  100  100  100                             Dioctylphthalate                                                                          50     50     50   50   50   50   50                              Epoxidized Soya                                                                           5      5      5    5    5    5    5                               Oil                                                                           Stabilizer  3      3      3    3    3    3    3                               (Therm-Chek 120)                                                              Alumina     30     30     30   30   30   30   30                              Trihydrate                                                                    Antimony Oxide                                                                            0      1.25   2.5  3.75 5.0  7.5  12.5                            4ZnO.B.sub.2 O.sub.3.H.sub.2 O                                                            0      1.25   2.5  3.75 5.0  7.5  12.5                            ______________________________________                                                    Formulation No.                                                                 8       9       10    11                                        ______________________________________                                        GEON 30 (PVC) 100     100     100   100                                       Dioctylphthalate                                                                            50      50      50    50                                        Epoxidized Soya                                                                             5       5       5     5                                         Oil                                                                           Stabilizer    3       3       3     3                                         (Therm-Chek 120)                                                              Alumina       30      30      30    30                                        Trihydrate                                                                    Antimony Oxide                                                                              0       2.5     7.5   12.5                                      4ZnO.B.sub.2 O.sub.3                                                                        0       2.5     7.5   12.5                                      ______________________________________                                    

The formulations were pressed into test articles and the fire retardancydetermined by measuring the limiting oxygen index (LOI) according to theprocedure of ASTM D 2863. The results are given in Table II.

                  TABLE II                                                        ______________________________________                                               Formulation No.                                                                          LOI                                                         ______________________________________                                               1          25.6                                                               2          28.1                                                               3          30.0                                                               4          32.5                                                               5          32.9                                                               6          35.8                                                               7          36.1                                                               8          25.6                                                               9          29.8                                                               10         32.3                                                               11         33.9                                                        ______________________________________                                    

Smoke suppressant efficacy was determined by measuring the specificoptical density Dm(Corr) of smoke generated by burning test samplesusing a NBS smoke chamber according to the procedure of ASTM E662. Theresults are set forth in Table III.

                  TABLE III                                                       ______________________________________                                        Formulation No.  Dm (Corr)                                                    ______________________________________                                        1                184                                                          2                157                                                          5                122                                                          7                126                                                          ______________________________________                                    

EXAMPLE VII

The compounds of this invention are also useful as anti-corrosionadditives for polymeric coating compositions. To test the hydrated zincborate 4ZnO.B₂ O₃.H₂ O, cold rolled steel test panels coated with anunoptimized medium oil alkyd primer paint containing this material weresubjected to continuous salt spray conditions according to the standardtest method ASTM B117. Negative controls, consisting of identical steelpanels coated with equivalent paint formulations without the zincborate, were treated simultaneously under the same conditions. Thepigment volume concentrations of the two test paints were maintained atthe same level by adjusting the volume of extender, calcium carbonate,used in the formulations. The test paint formulations are given in TableIV. The test paints were each applied to three replicate degreased,burnished standard cold rolled steel test panels using a standarddrawdown bar to provide coatings having a dry film thickness of 2 mils.After drying for one week at room temperature the panels were scribedwith a St. Andrew's cross and placed into a standard salt spray chamber.After 400 hours of continuous salt spray exposure at 95° F. the panelswere removed and evaluated for blistering according to the standardmethod ASTM D714. The paint film was then stripped using paint removerand the metal surface was evaluated for rusting according to thestandard method ASTM D610. The results of these tests are given in TableV.

                  TABLE IV                                                        ______________________________________                                                           Formulation No.                                                               I     II                                                   ______________________________________                                        Pigment Grind:                                                                Aroplaz 1082-M-50 alkyd resin                                                                      265.3*  265.3                                            Bentone SD-1         4.6     4.6                                              Mineral Spirits      42.4    42.4                                             Calcium Carbonate    88.0    159.8                                            Titanium Dioxide     212.1   212.1                                            Mica                 27.0    27.0                                             4ZnO.B.sub.2 O.sub.3.H.sub.2 O                                                                     100.0   --                                               Grind at high speed for 15 minutes and letdown                                at slow speed.                                                                Letdown:                                                                      Aroplaz 1082-M-50    159.1   159.1                                            Mineral Spirits      140.1   140.1                                            4% Calcium Drier     6.4     6.4                                              6% Cobalt Drier      1.3     1.3                                              6% Zirconium Drier   3.8     3.8                                              Antiskinning Agent   1.3     1.3                                              ______________________________________                                         *parts by weight                                                         

                  TABLE V                                                         ______________________________________                                               Blistering   Rust                                                      Formulation                                                                            Frequency.sup.1                                                                          Size.sup.2                                                                            Grade.sup.3                                                                          Undercutting                               ______________________________________                                        I        F          S8      9      Slight                                     (Zinc Borate)                                                                 II       MD         S2      3      Considerable                               (Control)                                                                     ______________________________________                                         .sup.1 ASTM D714 blister frequency:                                           F = Few                                                                       M = Medium Dense                                                              .sup.2 ASTM D714 rates blister size on a numerical scale of 0 to 10 in        which blister increases from S10 (no blisters) to SO (very large              blisters).                                                                    .sup.3 ASTM D610 rust grade 9 corresponds to minute rusting (less than        0.03% of the surface): rust grade 3 corresponds to rusting over               approximately one sixth of the surface.                                  

The very low water solubility of the compounds of this invention makethem especially useful for applications in plastics and coatings wherevery low leaching rates are desirable, such as under conditions ofsubmersion in water or exposure to weathering.

Various changes and modifications of the invention can be made and, tothe extent that such variations incorporate the spirit of thisinvention, they are intended to be included within the scope of thefollowing claims.

What is claimed is:
 1. The method of producing a crystalline zinc borateof the formula 4ZnO.B₂ O₃.H₂ O which comprises reacting zinc borate ofthe formula 2ZnO.3B₂ O₃.3.5H₂ O with zinc oxide in boiling water.
 2. Themethod of producing a crystalline zinc borate of the formula 4ZnO.B₂O₃.H₂ O which comprises reacting sodium tetraborate with a zinc salt inboiling water.
 3. A fire-retardant polymer composition comprising anorganic polymer and a fire-retarding amount of zinc borate having theformula 3.9-4.1(ZnO)0.9-1.1(B₂ O₃)0.8-1.2(H₂ O).
 4. A fire-retardantpolymer composition according to claim 3 in which said zinc borate is acrystalline hydrated zinc borate of the formula 4ZnO.B₂ O₃.H₂ O.
 5. Afire-retardant polymer composition according to claim 3 in which saidorganic polymer is processed at temperatures in the range of about300°-400° C.
 6. The method of producing fire-retardant polymercompositions which comprises incorporating a fire-retarding amount of azinc borate having the formula 3.9-4.1(ZnO)0.9-1.1(B₂ O₃)0.8-1.2(H₂ O)in said polymer.
 7. A method of producing a crystalline zinc borate ofthe formula 4ZnO.B₂ O₃.H₂ O which comprises reacting zinc oxide withboric acid in a mole ratio of about 2:1 in an aqueous solution at anelevated temperature about the boiling point of the mixture, therebyforming said crystalline zinc borate and separating said crystallinezinc borate from said aqueous solution.
 8. The method according to claim7 in which a slight molar excess of boric acid is present and saidreaction is in the presence of previously prepared seed crystals of saidzinc borate.
 9. The method according to claim 7 in which said boric acidis added in at least two portions to said zinc oxide in boiling water,thereby maintaining the pH of the reaction mixture above about 5.5. 10.A method of producing a crystalline zinc borate of the formula 4ZnO.B₂O₃.H₂ O which comprises reacting zinc oxide with boric acid or saltthereof in a mole ratio of about 2:1 calculated as the boric acid, inwater at an elevated temperature about the boiling point of the mixturefor a period of time sufficient to form said crystalline zinc borate4ZnO.B₂ O₃.H₂ O.
 11. The method according to claim 10 in which saidreaction takes place at the boiling temperature of the water.